A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The high accuracy and high resolution currently aimed at in lithography may require an accurate positioning of parts of the lithographic apparatus such as the reticle stage to hold the patterning device (e.g. mask), the projection system and the substrate table to hold the substrate, with respect to each other. Apart from the positioning of e.g. the reticle stage and the substrate table, this also poses requirements on the projection system. The projection system in current implementations may consist of a carrying structure, such as a lens mount (in case of transmissive optics) or a mirror frame (in case of reflective optics) and a plurality of optical elements such as lens elements, mirrors, etc. In operation, the projection system may be subject to vibrations due to a plurality of causes. As an example, movements of parts in the lithographic apparatus may result in vibrations of a frame to which the projection system is attached, a movement of a stage such as the substrate stage or the reticle stage, or accelerations/decelerations thereof, which may result in a gas stream and/or turbulence and/or acoustic waves affecting the projection system. Such disturbances may result in vibrations of the projection system as a whole or of parts thereof. By such vibrations, displacements of lens elements or mirrors may be caused, which may in turn result in an imaging error, i.e. an error in the projection of the pattern on the substrate.
Commonly, a damping system is provided to dampen vibrations of the projection system or parts thereof. Thereto, a damping system may be provided as known in many forms. In a configuration, the damping system may include an interface damping mass to absorb vibrations of at least part of the projection system, as well as an active damping subsystem to dampen a vibration of at least part of the interface damping mass. With this, the interface damping mass is connected to the projection system, and the active damping subsystem is connected to the interface damping mass. In this document, the term active damping system is to be understood as a damping system which includes a sensor to detect an effect of a vibration (e.g. a position sensor, velocity sensor, acceleration sensor, etc) and an actuator to act on the structure to be damped or a part thereof, the actuator being driven by e.g. a controller in dependency of a signal provided by the sensor. By driving the actuator in dependency of the signal provided by the sensor, an effect of vibrations on the projection system and/or the interface damping mass connected therewith, may be reduced or cancelled to a certain extent. An example of such an active damping system may be provided by a feedback loop: the sensor to provide a position quantity, such as a position, speed, acceleration, jerk, etc of the interface damping mass or a part thereof, the controller being provided with the position quantity and generating a controller output signal to drive the actuator, the actuator in turn acting on the interface damping mass or the part thereof so that a feedback loop is provided. The controller may be formed by any type of controller and may be implemented in the software to be executed by a microprocessor, microcontroller, or any other programmable device, or may be implemented by dedicated hardware.
It is desirable to stabilize the feedback loop, i.e. to achieve a frequency behavior of the feedback loop wherein internal resonances are prevented. At the same time, a high bandwidth of the active damping system is desired, as a high bandwidth of the active damping system will allow to suppress vibrations within such high bandwidth. Due to the ever increasing demands on speed of the lithographic apparatus, movements in the lithographic apparatus tend to take place at a higher speed and consequently involving faster transients, which may result in a generation of vibrations at increasingly higher frequencies. Therefore, a demand comes forward towards a higher bandwidth of the active damping system. For the damping system to work properly, the interface damping mass needs to behave like a rigid body over a very large frequency range. However, the interface damping mass, for example a solid block of steel or other material, already has internal dynamic behavior. If for example the interface damping mass weighs 10 kg, then a minimum internal resonance frequency of the interface damping mass may lie around 15 kHz. This resonance frequency is visible in the transfer functions of the entire damping system, and limits the achievable performance.
The projection system housing may due to external forces, such as forces caused by mechanical vibrations, acoustics, air flows, be excited at the eigenfrequencies of one or more lens elements arranged in the projection system housing. The resulting movements of the projection system housing will be taken into account in the servo control loop of the substrate and/or patterning device support, which attempts to position the support with respect to the projection system housing. However, the frequency with which the projection system housing vibrates may be too high for the support to follow, hence inducing imaging errors because the relative position of the support and the projection system housing is not according to the desired position. Alternatively, an increased settling time of the servo systems could be used to wait until the projection system housing stops vibrating, which settling time would have to be large since these lens elements are mounted in the projection system housing with a mounting having a low damping. As a result the overall throughput of the lithographic apparatus is negatively influenced.